This invention relates to a sheet stacking apparatus, and is particularly concerned with such an apparatus which comprises a device for feeding sheets or sets of sheets from an exit location into a catch tray positioned below the exit location, and means for supporting sheets or sets fed into the catch tray such that further sheets or sets fed from the exit location will encounter substantially constant stacking conditions.
Sheet stacking apparatus are used in many situations where sheets or sets of sheets are fed out from, for example, printing, photocopying or duplicating machines. In addition to such machines in which single sheets are fed out, various finishing stations, such as binding or stapling stations, may deliver bound or stapled sets of sheets. In machines and finishing stations of the kind mentioned, the sheets, usually of paper, are often passed over or between rollers, and are often heated, which gives the sheets or sets fed out a strong tendency to curl, or even to roll up. Modern machines can often produce large volumes of fed-out sheets or sets, and relatively deep catch trays are needed to accommodate their output. In the absence of anything to prevent it, a first sheet or set being fed from an exit location into a deep catch tray will have to be fed through or fall a much greater distance before it comes to rest on the base of the catch tray than if the catch tray were, for example, almost full with previously-fed sheets or sets. In such circumstances, there is a tendency for the first-fed sheet or sets to curl, or roll up, to the great inconvenience of the user of the machine.
A previous solution to this problem has been to use a catch tray with an elevating base, the base being arranged to keep the top of the stack held in the catch tray at a substantially constant height. One example of such an arrangement may be seen in U.S. Pat. No. 3,458,187 in which a pivotally mounted base is urged upwards by a spring, the spring being compressed in accordance with the weight of sheets held on the base. This solution, although effective, is expensive to implement compared with a simple, fixed catch tray which does not have an elevating base.